Frequency modulated oscillating system



G. T. ROYDEN FREQUENCY MODULATED OSCILLATING SYSTEM Filed Sept. 15, 1945 March 1, 1949.

INVENTbR. 6 1;? 90 622 ATTORNEY Patented Mar. 1, 1949 UNITED STATE S PATENT OFFICE FREQUENCY MODULATED OSCILLATING SYSTEM George '1. RoydemSouthOrange, N. 3., assignor to Federal Telephone and Radio Corporation, New York, N. Y., a corporation of Delaware Application September 15, 1945, Serial No. 616,622

18"Claims. 1 This invention relates to .an electrical apparatus and particularly to a frequency modulated oscillating system. In oscillating systems of the .prior art wherein modulation'is effected by varying the frequency of a carrier, considerable difficulty has been experienced in maintaining the carrier at amean value. In addition, difficulty has been experienced in providing simple circuits having a relatively low number of amplifiers and vacuum tubes.

Frequency modulation is generally used at high frequencies of theorder of 30 megacycles and higher. In-systems'operating at such frequencies, complexity of apparatus and circuit 'for obtaining frequency stability. The line itself is used as means for introducing feedback inan oscillator. Inaddition, the variation of -impedance at one end of the line is used to control the effective electrical length of the feedback circuit.

In general, theinvention provides atank circuit into which an .oscillator feeds power for maintaining oscillations. Coupled to the tank and thus affecting the tank constants is one end of a long transmission line. The transmission line may be any one of a number of different types, and preferablyihas an electrical length of the order of five or more wave lengths. The longer the line, the more stable the system. In accordance with theinvention, the other end of the line is connected .throughcoupling'impedances to the input of an oscillator, the output of the oscillator being connected to the tank circuit. The coupling impedance between the tubein'put and end of theline'may be varied or'modulated, and thus functions to control the frequency of oscillation.

As is well known, most oscillators tend to'maintain themselves in oscillation. For the ordinary three-element vacuum tube type of oscillaton'the condition for oscillation required substantially 1'80 degrees difference in .phaseibetween output .an'd 'input. "The 180 degree difference'in'phase .minated .by a vacuum tube.

surge impedance of the .line. .of this added reactance due to modulation will 2 may have. any additional integral numbers of 360 degrees.

A vacuum-tube oscillating system as a 'rule adaptsitself readily to circuit changes. Advantage of this istaken to "control the oscillation frequency by changing the equivalent electrical length of a section of artificial line which is in series with a real line .in the feedback circuit. It is understood,-of course, that an 'oscillatingsystern of this characterhas frequency limitations.

However, withinwide ranges, a system of this charactermay be made to vary its frequency of oscillation over a substantial range with reference to a mid or unmodulated carrier frequency.

In another embodiment of my invention, coupling impedance'between the end of the long line and the oscillator tube includes a short section of transmission-line to which there is connected another short section of transmission line whose lengthisnot an integral number of one quarter wavelengths, this last mentioned line being ter- In accordance with well-known line theory, the reflected impedance of .suchaline will be reactive when the line is terminated with a resistance not equal to the Hence, variation .cause the oscillator .to shift to another frequency for which'the .180- degree (or odd multiple thereof) .phasedisplacement in the oscillator feedback .circuitiissatisfied.

.Inorder to fully disclose the invention herein,

several. exemplary embodiments are hereby given,

it beingunderstood that these embodiments may undergo numerous modifications as hereinafter Referring now to the drawings,-Figure .1. shows .a circuit diagram of .a system embodying this invention. Figure 2 shows a modification of the r-shown. Vacuum tube lil-isof the three-element -rtype,although other types having-more electrodes .maycbenused. Tube H has cathode .ll suitably energized: by any meanssuch as battery :12. :It is understood that :cathode H may be' of the indirectly :heated type whose heater may beenergized by alternating current. The showingof the directly heated cathode with a battery-is merely symbolic. Cathode H is connected :at junction l'3 to grounded conductor M. It is understood that the cathode ground may beseifectivelyilis- "posed at the center of the cathode: in any usual fashion as a center tapped resistor connected across the cathode.

Tube II! has anode l6 connected through inductance H to junction l8. Junction I8 is connected through radio frequency choke l9 and conductor 20 back to positive terminal of a source of direct high potential 2|; This source is symbolically shown as a battery, although it is to be understood that any source of high potential may be used. Thus, rectified current at suitable potential with or without filteringmay be used. Under certain instances, it may even be possible to use high voltage alternating current at a relatively low frequency, such as 60cycles, and rely upon the rectifying action of the various tubes. The negative terminal of high voltage source 2| is connected to conductor l 4.

Connected between anode l6 and ground is tuning condenser 23, this condenser preferably being variable to tune the tank circuit. Between junction [8 and ground are condensers 24, 25 and 28 respectively connected in series. Condenser 24 is merely a blocking condenser while condensers 25 and 26form a radio frequency voltage divider.

Between condensers 24 and 25 is junction point 21 from which lead 28 is taken. Lead 28, together with grounded conductor l4, supplies energy to load 30. This load may be an antenna 7 veloped, no further details thereof are necessary.

Between condensers 25 and 26 is junction 3| to which conductor 32 is connected. Conductor 32is the high (R. F.) potential side of transmission line 33, which may be of any type desired. Thus, transmission line 33 may be of the openwire type with conductor 32 being one wire and a grounded Wire being the other part of the line.

Transmission line 33 may be, and preferably is, of the coaxial type consisting of an outer grounded sheath and a central conductor suitably supported by insulating means. The frequency range at which this system operates makes the use of a wave guide impractical, although there is no reason why it cannot be used.

Transmission line 33 has a length sufficiently great so that well-recognized phenomena of long lines come into play. While lines having a length of the order of a quarter wave length are considered long lines for certain purposes, transmission line 33 preferably has a length substantially in excess of one wave length and preferably in excess of five wave lengths. The effective length of transmission line 33 as is well known is a function of dielectric. Various means for increasing the electrical length of a line with respect to its physical length are well known and may be utilized here. It is preferred, however, to have transmission line 33 provide efficient transmission of energy with a minimum of loss. It is even more important that the electrical length be little affected by temperature, humidity and other similar variables.

The exact. physical and electrical length of line 33 is of no great consequence. Thus, transmission line 33 may have an electrical length equivalent to say five wave lengths at a predetermined carrier frequency, In operation, however, the system will adjust itself so that the electrical length of line 33 has a definite departure from an integral odd number of half wave lengths, the actual number of halfv wave lengths being dependent on the tuning of the input and output circuits. The departure is dependent on the phase shift in the modulation network.

Transmission line 33 terminates in a coupling condenser 35, this condenser being connected across the line to ground, Coupling condenser 35 has its high potential terminal 36 connected to radio frequency inductance 31. Inductance 31 has terminal 38 connected to tuning condenser 39, this condenser'being connected between terminal 38 and ground. Thus, condensers 35 and 39 and inductor 31 form an input tank circuit. In addition, terminal 38 is connected through condenser 40 to terminal 4]. Terminal 4| is connected through condenser 42 to control grid 43 of oscillator tube [0. Control grid 43 has grid leak 44 connected between it and ground. This grid leak may have a source of negative bias, such as a bias battery (not shown), in series with it.

From terminal 4|, load resistor 45 goes to terminal 46. Terminal 45 is connected to anode 41 of one modulator tube 48. This modulator tube is shown as of the three-element variety, although it may have any number of electrodes desired. Tube 48 has cathode 49, this cathode being symbolically shown as energized by battery 50. Cathode 49 is connected to grounded wire [4 as far as radio frequency is concerned.

Vacuum tube 48 has control grid 52 connected to transformer secondary 53 of modulation transformer 54. Transformer secondary 53 continues on to grounded wire i4 and may have battery 55 in circuit therewith, this battery functioning to bias control grid 52 to a desired operating point.

Modulating transformer 54 has primary 56 fed by any suitable source of modulating potential. This modulating potential may be either voice or signal, and may range over the usual frequency values for modulating work. Thus, as a rule, modulating frequencies up to as high as 10,000 or even 20,000 cycles per second may be used. For television work, the modulating frequency may be higher. There is no reason why video bands may not be used for modulation.

Junction 46 is also connected to cathode 58 of a second modulator tube 59. Cathode 58 is also shown symbolically as being energized bybattery 60. Modulator tube 53 has its control grid 32 connected to point 53 on a voltage dividing circuitconsisting of resistances 64 and 55 connected between high potential lead 20 and ground M. It is understood that control grid 62 is thus biased at a suitable potential. Tube 59 has its anode 66 connected to high potential lead 20.

While modulating tube 48 may be used by itself without second modulating tube 59, it. is preferred to use the double tube arrangement shown. By virtue of this arrangement, greater impedance variations between junction 41 and ground due to modulating potentials in transformer 54 are possible.

In the eventthat grounded conductor i4 is merely a common terminalfor the system but is not itself a real ground, trouble may be experienced due to the unbalance of transmission line 33at the ends where connections are made to .conductor [4. In such case, quarter wave balancing sleeves at the ends of transmission line 33 may be'used, such an expedient being well known in the art. .By virtue of such quarter wave sleeves, the outer conductor of transmission line 33 may be readily grounded without throwing the .ends of the line out of balance.

Referring to the tank circuit, condensers and 25 form, as pointed out above, a radio =frequency voltage dividing circuit. The relative capacitances of these twocondensers will be determined by the proportion of output energy 'tobe fed back andzalso by the characteristic impedance of the transmission line.

At the feedback end of the line, it will be noted "that coupling condenser 35 forms an energy transfer link between the feedback end of the transmission line and the network going to the input of tube iii.

Thus, line .33 at the feedback "end has its circuit completed by condenser 35. The network going to grid 13 forms several circuits. The first circuit includes condenser 35, inductor 3! and condenser 39, said condenser preferably being variable, and said circuit being substantially reso nant for the mid-frequency. The next circuit comprises condensers Mi and 12 together with resistors M and 45, also -the internal impedances of vacuum tubes '28 and 59. Thus, the impedance across condenser .35 as seen from the trans-- mission line includes the modulator tubes and grid resistor it.

By proper design of the coupling condenser, characteristic impedance of line 33, value of resistor M and dynamic plate resistance of modulator tubes 38 and 59, it is possible to match line 33 at the feedback end so that at carrier frequency (in the absence of modulation on tubes 48 and 59) line 33 is terminated substantially by a pure resistance whose value equals the characteristic impedance of line 33. Under such conditions, line 33 is non-resonant, hassubstantially no reflections and merely acts as a conduit for feeding back some energyin the tank circuit.

Upon the presence of modulating voltage on the control grid 52, the plate resistance is changed. If grid 52 goes positive, the resistance drops. This is true of both tubes (58 and .59. For radio frequency currents, these tubes .are effectively in parallel between junction 46 and ground. Hence, the modulating system may be considered as a variable resistance between junction ll and ground whose value varies from practically infinity to about the value of resistance 35. This controls the phase shift .in the modulation network between definite. limits. The lower limit is. determined by the capacitance of condensers ill and 42 in series with each-otherand resistor 44. The upper limit is the sum of the :phase shifts in the circuits comprising condenser 43 with resistor 45 considered as having one end grounded and condenser t2 with resistor i l.

A change of phase'shift causes the oscillator to immediately assume another frequency such that there is an exact multiple of 360 degree phase displacement in the oscillatorcircuit in- .cluding the tube itself and the complete feedback circuit. In other words, the variation of modulator resistance in the :phase shift network :acts as 'a variable artificial line element and causes the electrical line "length of the feedback :circuit tovary atmodulating frequency.

It is evident that the tank circuit into which vacuum tube oscillator Ii] feeds may take :on a

'variety of forms. Thus, the use of. a transmission "2, a modified system is shown wherein 'the tank circuit consists "of lengths of transmission line rather than conventional inductances and capacitances.

'Thus, referring to Figure 2, vacuum tube H) has its anode i6 connected to a short length of transmission line 'm. Transmission line 10 has the outer sheath-grounded to cathode H, while the inner conductor, assuming that this transmission line is of the coaxial type, is connected to junction H. Junction ll corresponds to junction 'IBcf Figure 1, and is connected through radio frequencychoke 19 in the same way as in Figure 1.

From junction point H, a connection goes through blocking condenser 12 to junction 13. From junction l3,-one length of transmission line 74 goes to junction l5. Junction W has alsoconnected'thereto a short length of transmission line "76 to which load '36 may be coupled. 'From junction point '55, a short length-of line 18 extends. Line '58 has a short circuit at 19. From junction 15, a connection is made to transmission line '33 back to the input network in a manner similar to that shown in Figure 1.

The total length of line portionslt'i, T4 and 18 isof the order of one-quarter wave length. Line 78 has such an electrical length as to provide the proper feedback *voltage to line 33. The lengths of line "l4 and'lfl are sochosen as to provides. proper impedance match for transmission line it going to load 3D. Substantial leeway in lengths 1B and M are possible. Thus, if line 16 is one-half a wave length long, it-will merely act as a one-to-one transformer. On theother hand, if line 16 is one-quarter wave length, then the relationship of the impedances at the two ends of the line will be in accordance with the wellknown transmission line formula. Other lengths of line for "56 "may be chosen. Thus, if for some reason lines 18 and 14 must have a-certain-electrical length, then line It may be adjusted to provide the proper impedance match to load '30. This load may bean antenna system suchas in Figure 1.

By having the electrical lengths of line 1B, "14 and 18 small, it is possible to permit the electrical length of these lines to vary substantially due to the action of the oscillator changing-its frequency. Thus, a quarter-wave line broken up as shown has no great inherent frequency stability, and'may be oscillated over'a substanjunction 83, a connection goes to the inner conductor of transmission line 33'. Another connection goes to matching stub 84 having the end short-circuited at 35. This matching stub is the terminating impedance for transmission line 33, and the length of stub 84 is chosenaccordingly.

From junction .83, another connection goes to transmission line 87, which .line is connected through :blocking condenser'iiBtto junction-46' of modulator tubes "48 and :59. The outer grounded parts :of the various lengths f. line are connected together as shown. Inasmuch as the rest of the modulator circuit is the same as Figure 1, a detailed description thereof is deemed to be unnecessary.

Matching stub 84 and line 82 together total approximately one-quarter of a wave length. The electrical length of line 81 is approximately oneeighth Wave length. The drawing does not show any relative line lengths. I

It is preferred to so bias the grids of modulator tubes 48' and 59, that, under normal conditions, the radio frequency parallel resistance of these modulator tubes will be substantially equal to the characteristic impedance of transmission line 87. As the resistance of the modulator tubes changes due to the modulating potentials on the grids, the termination of line 8'! will no longer be perfect. The variation'in resistance at junction 46' will be transmitted as a variation of reactance to junction 83, since line 81 is not an integral number of one quarter wave lengths long. The sign and magnitude of the reactance introduced at junction 83 will depend on the direction and amount of departure of the parallel resistance of the modulator tubes from the characteristic impedance of line 81. It is the introduction of this reactance during modulation that causes the oscillator to shift its frequency. The types of modulator tubes and their characteristics will therefore be a factor in the length of line 87.

The use of transmission line sections as shown in Figure 3 may, under some conditions, be preferable to the use of conventional circuit elements as shown in Figure 1. Thus, transmission line elements as a rule have greater transmission efficiency and sharper frequency response char--. acteristics than is true of the ordinary circuit elements. In addition, if the frequency is high enough and the spacing between parts is great enough, it is evident that some electrical length of line will be necessary in mere physical connections. Where conventional circuit elements such as condensers and. inductances are utilized, the

added effects due to substantial electrical lengths of line may be objectionable. In this particular system as shown in Figure 3, the physical and electrical length of line is used.

It is evident that Figures 2 and 3 may be combined to form a system wherein the output circuit of oscillator tube l and the input network of oscillator tube l0 may be as disclosed in Figures 2 and 3 respectively. Thus, by combining these two figures, a substantially complete system utilizing lengths of line as circuit elements may be used.

High voltage source 2| generally has a low impedance for high frequency alternating currents. In the event that some sources do not have such a low impedance, a by-pass condenser across the high voltage source may be used.

Other modifications will be evident to those skilled in the art. As one example, phase shift condensers 40 and 42 may be replaced by in ductors where blocking effects are not needed. The entire system will readily adjust itself to this change. This is particularly true when it is remembered that a transmission line provides inductive or capacitive reactance depending upon the exact length. Hence, the transmission can accommodate itself to such a change.

The precise location where variable phase shifting is provided in the entire oscillating circult is not important. Hence the phase shifting circuit and modulator for action on the phase shift circuitmay be moved bodily to the output side of vacuum tube l0, particularly between anode l6 and the tank circuit, consisting of inductor l1 and condensers 23 to 26 inclusive. Such an arrangement may be desirable where the oscillating system is heavily loaded and might cease oscillating. The phase shifting in the output of tube l0 would thus take place at a high power level. The feedback to the input could therefore be accurately stabilized to insure maintenance of oscillation.

It is also possible to use the modulator of Fi ure 3 in the output side of tube It) by connecting line 81 to junction 15 of Figure 2 instead of as shown in Figure 3. Thus, the modulator would be at the input to line 33'.

In all cases, the long feedback transmission line changes its electrical length with changing impedance conditions at the ends to provide nonresonant operation. It is possible that with sudden change of impedance at one end of the line, reflections occur and the line adapts itself as much as possible to eliminate reflections. While some reflections in the line may be present at all times, they are generally negligible. In fact, at high frequencies, reflection factors of 1.2 are regarded as an indication of practically nonresonant operation. Obviously, feedback energy must reach the input to amplifier ID or H) as the case may be in suificient quantity and in correct phase to maintain oscillation.

Thus the invention provides a tank circuit. long transmission line and a phase shiftin system forming a Wave transmitting network between the output and input of an amplifier. The maintenance of suitable resonance and impedance conditions promotes the natural tendency of the system to oscillate. The control of the phase shifting portion of the entire system thus varies the oscillation frequency. For matching impedances and stability, an input tank circuit may terminate the end of the line going to the amplifier input. Both tanks will be tuned to the same frequency and provide proper termination impedances for the line.

It is understood that the oscillation frequency of the entire system Will be high in comparison to the frequency difference between the highest and lowest available frequency. As a rule, frequency variations of substantially less than 1% may be tolerated. The higher the oscillation frequency, the smaller is the percentage of frequency variation possible.

What is claimed is:

1. An oscillating system whose frequency may be varied in accordance with modulating potentials, said system comprising a long transmission line, a tank circuit coupled to one end of said line, a vacuum tube having an input and an output circuit, means coupling said output circuit to said tank circuit, an impedance network coupled between the other end of said line and the input circuit of said tube, and means for varying the impedance of said network in accordance with desired signals whereby the frequency of oscillations in said tank circuit will be varied.

2. A variable frequency oscillating system comprising a vacuum tube having input and output circuits, a high Q network in the output circuit, a low Q network in the input circuit, a long transmission line coupling said two networks and means for varying the impedance of said low Q network in accordance with signals to be transmitted.

3. A variable frequency oscillating system comprising a vacuum tube having input and output circuits, a tank circuit connected to the output circuit, a transmission line having a length substantially greater than one wave length having one end coupled to said tank circuit, said coupling being such that the potential impressed on said one end of the line is substantially less than the maximum potential generated in said tank circuit, an impedance network connected to said input circuit, said impedance network including a variable resistance, means for coupling the other end of said transmission line to said variable resistance, and means for varying said variable resistance in accordance with signals to be transmitted.

4. A variable frequency oscillating system comprising a vacuum tube having an input and an output circuit, a tank circuit including inductance and capacitance, said latter being present in the form of at least two series-connected condensers connected across said output circuit, a transmission line having a length of the order of five wavelengths connected across one of said condensers, an impedance network connected to the said input circuit, means for coupling the other end of said transmission line to said impedance network, and means for changing the impedance of said network as seen from said line in accordance with signals to be transmitted.

5. A variable frequency oscillating system comprising a vacuum tube having input and output circuits, a tank circuit including a plurality of sections of transmission line forming part of said tank, a long transmission line, said long line having a length greater than the total length of said line sections forming the tank circuit, means for coupling one end of said transmission line to said tank circuit, an impedance network coupled to said input circuit, means for coupling the other end of said line to said impedance network, and means for varying the impedance of said last-named network in accordance with modulating signals.

6. In a variable frequency oscillating system, a vacuum tube having input and output circuits, a tank circuit coupled to said output circuit, a load coupled to said tank, a long transmission line, means for coupling one end of said line to said tank, an impedance network coupled between the other end of said line and said tube input circuit, a modulating vacuum tube connected to said network so that its plate to cathode resistance is an element of said impedance network, and means for impressing modulating signals on the control element of said modulating vacuum tube.

7. The system of claim 6 wherein an additional modulating vacuum tube is provided, said tube having its cathode connected to the anode of the first modulating tube, said two tubes havin their space discharge paths in series for direct current or low frequencies but being in parallel for radio frequency currents in said impedance network.

8. In a variable frequency oscillator, a vacuum tube having input and output circuits, a tank circuit connected in the output circuit, said tank circuit having output terminals to which a load may be connected, a transmission line having a length of the order of at least five wave lengths, means for connecting one end of said transmission line to said tank circuit, an impedance ne work in the input circuit of said vacuum tube, means for connecting the other end of said line to said network and signal control means for varying the impedance of said network.

9. The system of claim 8 wherein said impedance network includes a pair of modulating tubes with their space current paths connected in series for direct current and in parallel for oscillating currents, and wherein means are provided for impressing modulating signals on the control grid of one of said tubes.

10. In a variable frequency oscillator, a threeelement vacuum tube having cathode, control grid and anode, a tank circuit including inductance and capacitance, connections between said anode, cathode and tank circuit whereby said vacuum tube may sustain oscillations in said tank circuit, an output circuit connected across said tank circuit, a transmission line having one end connected to said tank circuit, said connection being such that the voltage impressed on said line is substantially less than that existing at the output terminals, a grid resistance connected between the grid and cathode, a network connected to said grid resistance, a connection between the other end of said transmission line and said network, an impedance connected in said network and for alternating current disposed to shunt to said grid resistance, and means for varying the value of said impedance in accordance with signals to be transmitted, said network being so disposed that said grid resistance and variable impedance form a termination for said other end of the line.

11. The system of claim 10 wherein said line has an electrical length of the order of at least five wave lengths.

12. The system of claim 10 wherein said variable impedance comprises a pair of vacuum tubes each having cathode, control grid and anode, said vacuum tubes being connected so that the space discharge paths of said two tubes are in series for direct current but are substantially in parallel for alternating currents as part of the termination for said line.

13. In a variable frequency oscillator, a vacuum tube having a cathode, control grid and anode, a tank circuit connected to said anode, said tank circuit being adapted to have oscillations maintained therein, output terminals for said tank circuit, a source of high potential connected to the anode of said tube, a long transmission line having one end coupled to said tank circuit, an impedance network connected to said control grid, a connection between the other end of said line and said impedance network, a three-electrode vacuum tube forming part of said impedance network and connected eiiectively across said grid and cathode for high frequency currents, said three-electrode vacuum tube including a control grid, means for impressing modulating signals on said control grid, and a second three-electrode modulating tube connected between the anode of said first modulating tube and said high potential source, said second modulating tube having its cathode connected to the anode of the first modulating tube so that the space paths of said two tubes are in series for direct current from said high potential source and low frequency alternating currents, said high potential direct current source having a low impedance for high frequency alternating currents so that said two modulator tubes are substantially in parallel for high frequency alternating currents.

14. In a variable frequency oscillator, an oscillator tube having a cathode, control grid and anode, a tank circuit connected across said cathode and anode, said tank circuit having output terminals, a long transmission line having one end connected to said tank circuit, a grid resistance connected between the grid and cathode, a blocking condenser to said grid for isolating said grid resistance, an impedance network connected between said cathode and blocking condenser, means for connecting the other end of said transmission line to said impedance network, a low impedance source of high potential connected across said anode and cathode, a pair of modulator tubes, each tube having a cathode, control grid and anode, said tubes being connected so that their space current paths are in series across said high potential source, a connection from the junction between the anode of one modulator tube and the cathode of the adjacent modulator tube to said blocking condenser, and means for modulating the control grid of one of I said tubes in accordance with signals to be transmitte'd.

15.'The system of claim 14 wherein said tank circuit includes a plurality of sections of transmission line.

16. The system of claim 14 wherein said impedance network includes a plurality of portions cathode.

sections. a further transmission line coupled at the junction of the first and second line sections, a load terminating said further transmission line, a network connecting said cathode and grid of said vacuum tube, a connection to the other end of said main transmission line and said network, said network including an impedance terminating said other end of said main line, and means for varying the value of said impedance in accordance with modulating signals.

18. The system of claim 17 wherein said network includes a grid resistance connected across the cathode and grid of said tube, a blocking condenser, a section of transmission line between said blocking condenser and a terminal, a terminating line section connected to said terminal, and a section of transmission lin connected between said terminals and a variable resistance,

said variable resistance being connected between the end of said last-named section of line and the GEORGE T. ROYDEN.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,121,737 Hansell June 21, 1938 2,283,793 Cork et al. May 19, 1942 2,321,269 Artzt June 8, 1943 2,382,198 Bollinger Aug. 14, 1945 2,421,725 Stewart June 3, 1947 

